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PowerPoint® Lecture Slide Presentation by Vince Austin Human Anatomy & Physiology FIFTH EDITION Elaine N. Marieb Chapter 11 Fundamentals of the Nervous System and Nervous Tissue Part C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Coding for Stimulus Intensity • All action potentials are alike and are independent of stimulus intensity • Strong stimuli can generate an action potential more often than weaker stimuli • The CNS determines stimulus intensity by the frequency of impulse transmission Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Coding for Stimulus Intensity • Upward arrows – stimulus applied • Downward arrows – stimulus stopped Figure 11.14 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Coding for Stimulus Intensity • Length of arrows – strength of stimulus • Action potentials – vertical lines Figure 11.14 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Absolute Refractory Period • Time from the opening of the Na+ activation gates until the closing of inactivation gates • The absolute refractory period: • Prevents the neuron from generating an action potential • Ensures that each action potential is separate • Enforces one-way transmission of nerve impulses Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Absolute Refractory Period Figure 11.15 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Relative Refractory Period • The interval following the absolute refractory period when: • Sodium gates are closed • Potassium gates are open • Repolarization is occurring • The threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Conduction Velocities of Axons • Conduction velocities vary widely among neurons • Rate of impulse propagation is determined by: • Axon diameter – the larger the diameter, the faster the impulse • Presence of a myelin sheath – myelination dramatically increases impulse speed Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Saltatory Conduction • Current passes through a myelinated axon only at the nodes of Ranvier • Voltage regulated Na+ channels are concentrated at these nodes • Action potentials are triggered only at the nodes and jump from one node to the next • Much faster than conduction along unmyelinated axons Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Saltatory Conduction Figure 11.16 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Multiple Sclerosis (MS) • An autoimmune disease that mainly affects young adults • Symptoms include visual disturbances, weakness, loss of muscular control, and urinary incontinence • Nerve fibers are severed and myelin sheaths in the CNS become nonfunctional scleroses • Shunting and short-circuiting of nerve impulses occurs • Treatments include injections of methylprednisolone and beta interferon Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Nerve Fiber Classification • Nerve fibers are classified according to: • Diameter • Degree of myelination • Speed of conduction Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Synapses • A junction that mediates information transfer from one neuron: • To another neuron • To an effector cell • Presynaptic neuron – conducts impulses toward the synapse • Postsynaptic neuron – transmits impulses away from the synapse Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Synapses Figure 11.17 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Electrical Synapses • Electrical synapses: • Are less common than chemical synapses • Correspond to gap junctions found in other cell types • Contain intercellular protein channels • Permit ion flow from one neuron to the next • Are found in the brain and are abundant in embryonic tissue Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chemical Synapses • Specialized for the release and reception of neurotransmitters • Typically composed of two parts: • Axonal terminal of the presynaptic neuron, which contains synaptic vesicles • Receptor region on the dendrite(s) or soma of the postsynaptic neuron Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Synaptic Cleft • Fluid-filled space separating the presynaptic and postsynaptic neurons • Prevent nerve impulses from directly passing from one neuron to the next • Transmission across the synaptic cleft: • Is a chemical event (as opposed to an electrical one) • Ensures unidirectional communication between neurons Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Synaptic Cleft: Information Transfer • Nerve impulse reaches axonal terminal of the presynaptic neuron • Neurotransmitter is released into the synaptic cleft • Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron • Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Synaptic Cleft: Information Transfer Figure 11.18 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Termination of Neurotransmitter Effects • Neurotransmitter bound to a postsynaptic neuron: • Produces a continuous postsynaptic effect • Blocks reception of additional “messages” • Must be removed from its receptor • Removal of neurotransmitters occurs when they: • Are degraded by enzymes • Are reabsorbed by astrocytes or the presynaptic terminals • Diffuse from the synaptic cleft Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Synaptic Delay • Neurotransmitter must be released, diffuse across the synapse, and bind to receptor • Synaptic delay – time needed to do this (0.3-5.0 ms) • Synaptic delay is the rate-limiting step of neural transmission Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Postsynaptic Potentials • Neurotransmitter receptors mediate changes in membrane potential according to: • The amount of neurotransmitter released • The amount of time the neurotransmitter is bound to receptor • The two types of postsynaptic potentials are: • EPSP – excitatory postsynaptic potentials • IPSP – inhibitory postsynaptic potentials Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Excitatory Postsynaptic Potentials • EPSPs are graded potentials that can initiate an action potential in an axon • Use only chemically gated channels • Na+ and K+ flow in opposite directions at the same time • Postsynaptic membranes do not generate action potentials Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 11.19a Inhibitory Synapses and IPSPs • Neurotransmitter binding to a receptor at inhibitory synapses: • Causes the membrane to become more permeable to potassium and chloride ions • Leaves the charge on the inner surface negative • Reduces the postsynaptic neuron’s ability to produce an action potential Figure 11.19b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Summation • A single EPSP cannot induce an action potential • EPSPs must summate temporally or spatially to induce an action potential • Temporal summation – presynaptic neurons transmit impulses in rapid-fire order • Spatial summation – postsynaptic neuron is stimulated by a large number of terminals at the same time • IPSPs can also summate with EPSPs, canceling each other out Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Summation Figure 11.20 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Neurotransmitters • Chemicals used for neuronal communication with the body and the brain • 50 different neurotransmitter have been identified • Classified chemically and functionally Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chemical Neurotransmitters • Acetylcholine (ACh) • Biogenic amines • Amino acids • Peptides • Novel messengers Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Neurotransmitters: Acetylcholine • First neurotransmitter identified, and best understood • Released at the neuromuscular junction • Synthesized and enclosed in synaptic vesicles • Degraded by the enzyme acetylcholinesterase (AChE) • Released by: • All neurons that stimulate skeletal muscle • Some neurons in the autonomic nervous system Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Neurotransmitters: Biogenic Amines • Include: • Catecholamines – dopamine, norepinephrine (NE), and epinephrine • Indolamines – serotonin and histamine • Broadly distributed in the brain • Play roles in emotional behaviors and our biological clock Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings